Many Metropolitan Area Networks (MANs) and some Local Area Networks (LANs) have a ring topology, such as a bridged, layer-2 network like the Ethernet. A problem with bus and ring networks like the Ethernet is the possibility of a single point of failure causing the system to breakdown. A common solution is to design the network with redundant segments and loops so that there is more than one route to each node in an approach that is similar to that employed in a Synchronous Optical NETwork (SONET), a layer-1 technology that was developed in the mid-1980s for the public telephone network and adopted by the American National Standards Institute (ANSI) as a standard for fiber optic networks. Redundancy and loops can, however, present another problem in which a broadcast packet or an unknown unicast packet results in a broadcast storm where each node receives and rebroadcasts the packet causing potentially severe network congestion.
One way of preventing broadcast storms and other unwanted side effects of looping is to use the Spanning Tree Protocol (STP), which has been standardized in the 802.1D specification by the Institute of Electrical and Electronic Engineers (IEEE Std. 802.1D-1998, IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Common specifications). With STP, the bridges in the network dynamically calculate an optimum subset of the topology that is loop-free and allows a path to each node. Alternate paths are blocked and unblocked as necessary to keep the routes of communication open when a fault occurs.
Another way of preventing broadcast storms and other unwanted side effects of looping is to use the Ethernet Automatic Protection Switching (EAPS) protocol, described in an informational Request For Comment (RFC) 3619 in October of 2003 as well as in U.S. Pat. Nos. 6,766,482 and 7,003,705, assigned to Extreme Networks, Inc., of Santa Clara, Calif., the assignee of the present application. Among other advantages, EAPS offers improved recovery times over STP, and provides LANS and MANS having a ring topology with resilience comparable to that provided by SONET rings at lower cost and with fewer constraints.
Regardless of whether STP or EAPS is used to prevent loops in a layer-2 network having a ring topology, it is sometimes difficult to insure that they are deployed correctly. For example, among other uses, the EAPS protocol is used to define a protection domain, referred to as an EAPS domain, using a control Virtual Local Area Network (control VLAN) on a single Ethernet ring. Because the configuration of a network in which EAPS is deployed may change, the integrity of an EAPS domain may be broken. It is also possible that the EAPS domain may not have been correctly defined. From a network management point of view, it may be necessary to periodically review EAPS domains to insure their proper operation.